246 EQUILIBRIUM BETWEEN CARBON DIOXIDE OF ATMOSPHERE 



we can now determine the value for the solubility product of calcium 



carbonate" Cca X Cco3 = KcaC03 



to which the precipitation of calcium carbonate is subject. As explained 

 above, the solution may be considered just saturated whenever the product 

 of the ion concentrations is equal to this constant. Excess of carbonic 

 acid increases the solubility through its hydrogen ions, which form bicar- 

 bonate ions HCO3' with the carbonate ions CO3" of the calcium carbonate. 



^^^'^ CcaXCCcOa-^XKcaCOa 



the solution is no longer saturated with calcium carbonate and the solid 

 carbonate will go into solution until we again have: 



C'ca X C'cOa = KcaCOs 



the concentrations C'ca and G'cOa being unequal now, calcium ions being 

 in excess. 



As stated on page 242, the solubility will depend on all four of our 

 fundamental equations I to IV (on p. 242), the functions being dependent 

 on the various constants. They may all be combined as follows: Divid- 

 ing equation III by IV, we have first: 



^ HCO3 ^ Ionization (24:) 



CcOs X ChoCOs K" Ionization 



We may substitute in this equation for Ccog its value as obtained from 

 equation I, viz. — ^^, and have 



KcaCOa 

 Ca 



Cca X C^HCOa K Ionization 



KcaCOs X ChsCOs ^' Ionization 



or 



CcaXC HCO:j t^ vy ^ Ionization fon\ 



7S = KcaCOs X -tTT; \^^) 



^H2C03 -"^ Ionization 



Now, when calcium carbonate is dissolved under the influence of excess 

 of carbonic acid in the absence of any other calcium salts, as was the case 

 in Schloesing's experiments, practically all of the calcium is present as 

 bicarbonate, the quantity of carbonate being minimal and quantitatively 

 negligible in comparison with the bicarbonate. Since 



Ca(HC03)2 ^ Ca" + 2HCO3' (26) 



the concentration of the calcium ions will be half that of the bicarbonate 

 ^^^^ Cca = iCHC03 (27) 



and by substituting this value in (25) we have 



C HCO3 oir \y ^ Ionization 



7^ = ^IVcaCOs X -T^, 



^HjCOs -"^ Ionization 



(28) 



For the concentration of the dissolved carbonic acid we have, according 



^^ CH.C03 = fe.asXP (29) 



<^Vide Bodlaender, loc. cit. 



